The present invention relates to image projection systems and, more particularly, to an image projection system of a spacer type which includes spacers to project an image obtained on an image field onto a screen correctly.
FIG. 1 of the accompanying drawings schematically shows an image projection system which includes a CRT (cathode ray tube) 1, a lens 2 and a screen 3 disposed with an illustrated positional relationship. The image projection system shown in FIG. 1 can project an image obtained on an image field onto a screen insofar as the CRT 1 and the lens 2 are properly placed relative to the screen 3. Therefore, if the lens 2 is disposed at a predetermined position (angle) determined by the image filed of the image projection system, such as the CRT 1 and the screen 3, it is possible to correctly project an image onto the screen 3. There is an image projection system of a spacer type which includes spacers interposed respective areas between the lens 2 and the image field of the CRT 1 to provide a necessary angle.
If the image projection system of the spacer type includes only one pattern of optical system, then it is sufficient that the image field of the CRT 1 and the lens 2 are fixed at a predetermined angle with rigid spacers. However, when an image projection system is set in an actual practice, a relationship between the surface of the screen 3 and the image field of the CRT 1 is changed variously with a combination of a screen size and an incident angle .alpha. (see FIG. 2A).
As shown in FIGS. 2A and 2B, reference symbol 6V assumes an angle (declination) in the vertical direction formed by the video screen of the CRT 1 and the end face of the lens 2 as seen from the side surface direction of the set position of the CRT 1 and the lens 2 (see FIG. 2A). Reference symbol 6H assumes an angle (declination) in the horizontal direction formed by the video surface of the CRT 1 and the end face of the lens 2 as seen from the upper surface direction of the set position of the CRT 1 and the lens 2 (see FIG. 2B). At that time, a center A of the screen 3 and an optical axis center B are not always coincident. In general, if the size of the screen 3 is large, then the angle .delta.H is small. If the size of the screen 3 is small, then the angle .delta.H is large. The incident angle .alpha. is changed variously in respective cases.
If all combinations of the screen sizes and the incident angles .alpha. are realized by the spacer, there should be prepared spacers of various proper sizes, which cannot be made in actual practice.
When the image projection system of the spacer type is used, predetermined angles can reliably determined in several fundamental optical systems but angles larger than the predetermined angles cannot be fine adjusted. Moreover, when angles other than those of the fundamental optical system are required, special spacers should be prepared additionally. If the thickness of the spacer itself is made variable, then the spacer itself becomes expensive. Also, those who adjust the spacer angle are required to become skillful.
Furthermore, if the lens and the video screen (CRT, etc.) are coupled with a gimbal structure having independent vertical and horizontal axes without using the spacer, accuracy between the respective axes and the bearings and a countermeasure for removing a vibration are low in reliability and disadvantageous from a cost standpoint. Moreover, upon adjustment, those who are skilled in adjustment are also required.